SOLID-STATE SI-29 AND B-11 NMR-STUDIES OF SOL-GEL DERIVED BOROSILICATES

Gels in the system SiO//2-B//2O//3 were prepared from metal alkoxides. The structural evolution of these gels as a function of thermal treatment was studied by high field **2**9Si and **1**1B NMR spectroscopy, using magic angle spinning. The results corroborate earlier work on this system using infrared spectroscopy to follow the formation of borosiloxane bonds during heat treatment. While incorporation of boron into the silica backbone is negligible in gels dried at room temperature only, thermal treatment drives the borosiloxane bond formation with removal of excess water. Initial boron incorporation involves the formation of terminal equals B-O-Si EQUVLNT groups with tetrahedral boron environments. Further heating causes the condensation of boron into symmetric trigonal equals B-O-Si EQUVLNT sites. Boron is fully incorporated by 450 degree C.